Tapas Kumar Pal et. al. / International Journal of Engineering and Technology Vol.2 (5), 2010, 329-335
Study of Various Tuning properties and Injection
Locking of Resonant-cap IMPATT oscillator
Tapas Kumar Pal#1 , J. V. Prasad#2 and J. P. Banerjee*3
#
Research Centre Imarat
Vignyana Kancha, Hyderabad – 500 069, INDIA.
*
Institute of Radiophysics & Electronics
University of Calcutta , Kolkata – 700009, INDIA.
Abstract - A coherent study of tuning properties of resonantcap IMPATT oscillator at Ka-band has been carried out by
mechanical and electronic means. It is experimentally observed
that output power of IMPATT oscillator passes through a
maximum, with an optimum combination of cap diameter and
cap height. An empirical relation is obtained between cap
diameter and wavelength of the optimised resonant cap
oscillator, which agrees well with theoretical relation. Electronic
tuning is carried out by varying the dc bias current with the
optimized cavity parameters. It is observed that the variations of
oscillation frequency and power output with dc bias current are
characterised by three ranges of bias current. The effect of
sliding short tuner position on the performance of the oscillator
at optimised condition has also been studied. Finally, the
injection locking of the free running oscillator has been studied
with a Ka-band signal generator as a reference source. The
injection locked IMPATT oscillator shows a good phase noise
performance.
Key words: Electronic tuning, Injection locking, Ka-band IMPATT
oscillator, Mechanical tuning, Phase noise.
I. INTRODUCTION
IMPact Avalanche Transit Time (IMPATT) devices based
on Silicon have already been established as powerful solidstate sources for generation of high CW and pulsed power at
wide range of microwave and millimeter wave frequencies
[1], [2]. These devices provide high oscillator output power
with high DC to RF conversion efficiency in Silicon
Monolithic Millimeter Wave Integrated Circuits (SIMMWIC)
[3]. The vast frequency range of operation with high power
capability of IMPATT device leads to its application as a
highly potential source for meeting the ever increasing
demand in mm-wave communication system. IMPATT
devices also find useful applications in tracking Radars. The
tunable bandwidth of IMPATT oscillator plays an important
role on the total system design. The tunable bandwidth of
microwave/millimeter wave IMPATT oscillator has been
investigated by several researchers and reported in the
published literatures [4-11]. Although some research work has
been reported in the field of resonant-cap IMPATT oscillators,
there remains a lot of scope for research on the detailed and
coherent study of their tuning properties specially at Ka-band.
Among all the window frequencies, 36 GHz in Ka-band is
gaining importance in recent years due to its wide application
in civil, industrial, medical and strategic fields. The purpose of
this paper is to present a detailed and systematic study of the
mechanical tuning properties of resonant-cap IMPATT
oscillator at Ka-band with the variation of cap diameter, cap
height and sliding short tuner position and also the electronic
tuning properties by varying the dc bias current.
Low phase noise oscillators are in great demand due to the
rapid growth of their application in millimeter wave
technology for both civilian and defence sectors. The phase
noise of IMPATT oscillator is an important parameter to
improve the overall system performance. In developing high
quality, low cost oscillators, a detailed and accurate study of
the phase noise performance of IMPATT oscillators is highly
needed for their operation in either a stable amplifier mode or
an injection-locked oscillator (ILO) mode [12]. The injectionlocked oscillator has the advantage of delivering higher output
power with high DC to RF conversion efficiency. These
oscillators are not only capable of providing higher gain over
a wider bandwidth but also adaptable to power combining.
Further they are generally more suitable because of their
higher efficiency and power output capability.
II. KA-BAND IMPATT OSCILLATOR
A resonant cap IMPATT oscillator consists of a resonant
cap structure under which the diode is embedded and these are
mounted in a rectangular wave guide. The cap along with the
broad surface of the wave-guide forms a localized radial
cavity around the diode. The millimeter wave power generated
by the diode is coupled to the main rectangular wave-guide
cavity through the vertical open edges of the radial cavity.
The resonant cap can be approximated as a radial transmission
line which acts like an impedance transformer between the
device and the load. The cross sectional view of IMPATT
oscillator used in the present study is shown in Fig.1 and the
photograph of the same is in Fig. 2 [13]. Experimental studies
of the tuning characteristics of resonant-cap oscillators have
been carried out by using silicon Ka-band SDR IMPATT
diodes having the following specifications: Frequency range =
35 – 42 GHz, Breakdown voltage = 45 Volts (max), Current =
150 mA (max), Power output = 100 mW (max). The block
diagram of Ka-band measurement test set up is shown in Fig.3
and a typical spectrum (36.75 GHz) of the oscillator is shown
in Fig.4.
Tapas Kumar Pal et. al. / International Journal of Engineering and Technology Vol.2 (5), 2010, 329-335
DC Bias
Ampere meter
Sliding short
tuner
IMPATT
Oscillator
Isolator
Voltmeter
Power meter
Directional
coupler
Attenuator
10 dB
Fig.1. Schematic diagram of a Ka-band resonant cap waveguide cavity
Spectrum
Analyzer
Fig.3. Block diagram of Ka-band measurement test set up
Fig.2. Photograph of resonant cap IMPATT oscillator
III. VARIOUS TUNING
A. Mechanical tuning
Mechanical tuning of a resonant cap IMPATT oscillator is
carried out by means of cavity tuning and also by varying
sliding short position. The cap diameter and cap height are
both varied for tuning the cavity. The variations of both the
oscillation frequency and output power with the cap diameter
and cap height have been experimentally determined. The
results are shown in Fig.5 and Fig. 6 respectively. The cap
diameter is varied from 3.4 mm to 4.5 mm and the
corresponding cap height from 1 mm to 2.0 mm to optimise
the oscillator performance. It is observed that output power
attains a maximum value of 90 mw when the cap diameter and
cap height are 4.0 mm and 1.6 mm respectively. The
oscillation frequency is found to vary from 36.00 GHz to
37.75 GHz during the process of cavity optimisation.
Fig.4. A typical spectrum achieved at Ka-band
For the variation of cap height from 1 mm to 2 mm in steps of
0.2 mm and it is observed that power level increases from 52
mw to a maximum of 88 mw for cap height of 1.6 mm and the
same decreases to 72 mw for cap height of 2 mm. The
corresponding oscillation frequency varies from 36.00 GHz to
37.5 GHz. The above studies have been carried out at a bias
current level of 145 mA and break down voltage of 30.5 Volt.
It is observed that the oscillation frequency decreases with the
increase of the cap diameter and reverse takes place for the
variation of cap height. The rate of decrease of oscillation
frequency with the increase of cap diameter for the Ka-band
IMPATT oscillator under study is found to be 1.5 GHz/mm.
With increasing cap height the oscillation frequency increases
at the rate of approximately 1.57 GHz/mm. It is also observed
that maximum output power at the desired frequency of 36.5
GHz is obtained when the cap diameter ranges from 3.8 – 4.2
mm and corresponding cap height ranges from 1.4 – 1.8 mm.
More precisely a suitable combination of cap diameter and cap
height i.e. 4.0 mm and 1.6 mm respectively leads to maximum
output power of 90 mw at a desired frequency of 36.5 GHz.
Thus the output power delivered by a resonant-cap IMPATT
Tapas Kumar Pal et. al. / International Journal of Engineering and Technology Vol.2 (5), 2010, 329-335
oscillator passes through a maximum at a particular frequency
for a given device with an appropriate combination of cap
diameter and cap height.
B. Relation between cap diameter and wavelength
The experimental results for the condition of optimum
performance of the resonant cap IMPATT oscillator are
related by the following approximate equation.
D

(1)
2
Where  is the operating wavelength and D is the cap
diameter. Thus a resonant–cap IMPATT oscillator can be
experimentally realised if the cap diameter is designed to be
half of the wavelength of desired frequency of oscillation for
optimum performance of the oscillator.
H  
jn
E 0 J n (k ) Sin(n )
k2
(2.b)
Hφ  
jωε
E 0 J n (kρ)Cos ( nφ)
k
(2.c)
Where k is the propagation constant, J n is the Bessel function
of the first kind and order n . The open circuited edge

condition requires that J n ( kr )  0 , where r is the radius of
the disk and the prime indicates differentiation with respect to
the argument. Thus for each mode of configuration the disc
radius corresponding to zeros of the derivative of the Bessel
function can be obtained which will lead to desired resonance
condition.
38.0
Output power
Frequency
90
70
37.0
60
36.5
50
36.0
Frequency [GHz]
Output power [mW]
Output power [mW]
37.5
80
80
37.0
36.5
70
36.0
60
Frequency [GHz]
38.0
Power
Frquency
90
37.5
35.5
50
35.0
1.0
1.2
1.4
1.6
1.8
2.0
Cap height [mm]
40
3.4
3.6
3.8
4.0
4.2
4.4
35.5
4.6
Fig.6. Variation of output power and Frequency with Cap height
(Cavity tuning)
Cap diameter [mm]
For
Fig.5. Variation of Output power and Frequency with Cap diameter
(Cavity tuning)
The experimental results obtained can be explained by
considering the electromagnetic fields within the resonant-cap
cavity. The resonant-cap cavity is formed between the disc of
the resonant-cap structure and the lower broad face of the
waveguide as shown in Fig.7. Such a cavity can be
approximately modelled as a cylindrical cavity, bounded at its
top and bottom by electric walls and on its sides by a magnetic
wall [14-16]. The electric field within the resonant-cap cavity
has essentially only a z-component, and the magnetic field has
essentially x and y-components. Because h<<g (guided
wavelength), the fields do not vary along the z direction, and
the component of the current normal to the edge of the cap
approaches zero at the edge. This implies that the tangential
component of the magnetic field at the edge is vanishingly
small. With these assumptions, the resonant cap can be
modelled as a cylindrical cavity, bounded at its top and
bottom by electric walls and on its sides by a magnetic wall.
Thus the fields within the resonant cap cavity, corresponding
to TMnm modes, may be determined by solving a cavity
problem. The fields corresponding to TM nm modes within the
cavity are given by [15].
E z  E 0 J n ( k )Cos ( n )
(2.a)
any given frequency, the dominant mode has
n  m  1 and it corresponds to the minimum radius of the

disc for resonance. The root of J n ( kr )  0 for dominant
mode is kr  1.841 . But k 
2

and 2r  D . The diameter
of the disc is obtained from the relation
D

 0.58
(3)
Tapas Kumar Pal et. al. / International Journal of Engineering and Technology Vol.2 (5), 2010, 329-335
38.0
Metal disc
37.5
80
Frequency [GHZ]
60
36.5
40
36.0
35.5
20
35.0
Power
Frequency
34.5
Lower broad face of the
wave guide (metallic)
-1
The dependence of the oscillation frequency and output
power on the sliding tuning short position has been studied
and the results are shown in Fig.8. Sliding tuning short
position has been varied from 0 to 9.7 mm in steps of 0.5 mm.
It is observed that RF power output varies from 0 mw to 88
mw and frequency varies from 34.50 GHz to 37.75 GHz. It is
also observed that the mechanical tuning is characterised by
an initial frequency jump followed by a sharp fall with a
subsequent rise of output power. A smooth mechanical tuning
range of 3.15 GHz with a centre frequency of 36 GHz is
obtained for a change of sliding short position over a range
of

2
. It is further observed that frequency jumps by 3 GHz
approximately. Also a sudden power gain of 70 mw is
achieved nearly at distances

2
and  between the sliding
short position and the diode plane. This experimental
observation agrees well with the reported results [5] and [7].
1
2
3
4
5
6
7
8
9
10
11
Fig.8. Variation of Frequency and output power with
sliding short tuner position (Mechanical tuning)
Output Power [mW]
37.2
Output power
Frequency
100
P
max
37.1
80
37.0
III
Power Curve
60
36.9
36.8
40
36.7
II
Frquency Curve
20
36.6
I
0
I
I
40
36.5
0
th
60
80
100
120
140
Frquency [GHz]
C. Tuning by Sliding Short
0
0
Sliding short tuner position [mm]
Fig.7. Resonant-cap cavity between the resonant-cap and
the lower broad face of the wave guide.
The theoretical relation (3) shows that the operating
wavelength  and the disc diameter D of the resonant-cap
are directly proportional and the constant of proportionality is
0.58.The empirical relation (1) derived from our experimental
results has a form similar to the theoretical relation (3) which
is found to be a close approximation to the results published in
[5], [8] and [17].
Output power [mW]
37.0
160
DC bias current [mA]
Fig.9. DC bias current Vs Output power and Frequency
(Electronic tuning)
D. Electronic tuning
Electronic tuning of the oscillator with the optimized cavity
parameters i.e. D = 4.0mm and h =1.6mm is carried out by
varying the DC bias current. DC bias current has been pushed
upto 160 mA keeping other parameters fixed. The variations
of output power and frequency with DC bias current are
shown in Fig.9. It is observed that the variations of oscillation
frequency and power output with dc bias current are
characterised by three ranges of bias current. In the first range
I, starting from threshold dc bias current for oscillation Ith ,
both the output power and frequency increase very slowly
with dc bias current. This is followed by a second range II of
dc bias current where the output power increases sharply to a
maximum value Pmax but the oscillation frequency increases
with dc bias current in an approximate linear nature. In the
third range III of dc bias current exceeding the current I0
corresponding to Pmax the power begins to fall gradually from
its maximum value but the frequency increases more sharply
compared to the second range, called burn out zone. The
Tapas Kumar Pal et. al. / International Journal of Engineering and Technology Vol.2 (5), 2010, 329-335
results agree well with the findings of Roy et al [11]. The
useful linear frequency range is approximately 34 MHz
corresponding to a change of dc bias current of 64 mA in the
second range II of dc bias current. The third range III of DC
bias current should normally be avoided to prevent the
electronic burn out phenomenon.
Frequency
Counter
Circulator
Directional
Coupler
Signal
Generator
IV. INJECTION LOCKED IMPATT OSCILLATOR
A. Injection Locking Model
The phenomenon of injection phase-locking may be used
to synchronize one or more oscillators to a lower power
master or reference oscillator and also to reduce some part of
the resultant FM noise power spectrum. The cavity oscillator
has been represented as a tank circuit where C0 is the
equivalent capacitance
of the cavity oscillator, L0 its
equivalent inductance, +G the conductance of the load, and -G
the equivalent negative conductance of the oscillator, equal in
magnitude to the conductance G of the load in the steady-state
condition [18]. The basic injection phase locked oscillator
model is shown in Fig.10 and the injection locking has been
done by a “reference” oscillator having good phase noise
performance.
Free Running
Ka-band IMPATT
oscillator
Directional
Coupler
Phase Noise
Measurement
System
Power Meter
Printer
Fig.11. Injection locked IMPATT Oscillator and
Phase noise measurement set up
Reference
Cavity Oscillator
(C0, L0,-G)
Circulator
Load
(+G)
Fig.10. Basic injection phase-lock circuit diagram
B. Injection Locking of IMPATT Oscillator
The indigenously developed Ka-band CW IMPATT
oscillator [13] is used as a free running oscillator with power
output 0 dBm and a Ka-band signal generator has been used
as a reference source. An injection signal from the signal
generator of very low power – 40 dBm is used to lock the free
running oscillator. Fig.11 shows a schematic of injection
locked Ka-band IMPATT oscillator.
Using the same
configuration the phase noise of Ka-band IMPATT oscillator
has been measured under injection locked condition. A snap
shot of the phase noise at different offset frequency has been
shown in Fig.12 by using a phase noise measurement system.
The measured values are tabulated in Table. 1 at various offset
frequencies.
Fig.12. Snap shot of Phase Noise at different offset frequency
(using phase noise measurement system)
Table: 1
Phase noise at different offset frequency
Offset Frequency
1 KHz
100 KHz
1 MHz
Phase noise
(Phase noise measurement
system)
-84.56 dBc / Hz
-93.75 dBc / Hz
-99.80 dBc / Hz
V. CONCLUSION
In this paper the authors have used three different methods
of mechanical tuning of wave guide mounted resonant-cap
Ka-band IMPATT oscillator. A wide tunable range of 3.15
GHz is achieved by sliding short and tunable ranges of 1.75
GHz and 1.5 GHz are obtained by varying cap diameter and
cap height respectively around the centre frequency of 36 GHz
Tapas Kumar Pal et. al. / International Journal of Engineering and Technology Vol.2 (5), 2010, 329-335
at Ka-band. For the design and development of millimeter
wave (Ka-band) IMPATT oscillator using resonant-cap wave
guide structure, the values of cap-diameter and cap height are
important parameters in deciding the output power as well as
the frequency. The authors have obtained an approximate
relation between cap diameter and wavelength under
optimised condition i.e., D =

2
which is very close to the
theoretical relation D/ = 0.58. The dependence of output
power and frequency with the sliding short position has been
demonstrated. In order to achieve the optimum condition the
sliding short position is to be tuned to a distance which are
multiples of

2
The authors have carried out electronic tuning by varying
the dc bias current and taking into account the values of the
optimized cavity parameters, i. e. cap diameter D = 4.0 mm
and cap height h = 1.6 mm. It is observed that the variation of
oscillation frequency and power output with dc bias current is
characterised by three ranges of bias current. In the first range
I, both the output power and frequency increases slowly, in the
second range II, both the output power and frequency increase
sharply. In the third range III, the output power decreases
where as frequency increases. The operating bias current of
the oscillator in the third range III, should normally be
avoided to prevent the burn out phenomenon of IMPATT
diode which occurs very frequently while tuning the IMPATT
oscillator.
Finally, the authors have demonstrated the injection
locking of IMPATT oscillator by using a signal generator as a
reference source with the corresponding phase noise
measurement. A phase noise of -84.56 dBc / Hz @ 1 KHz has
been realised from the Ka-band injection locked IMPATT
oscillator.
ACKNOWLEDGEMENT
1,2
The authors
are thankful to Shri. S.K. Ray, DS &
Director, Research Centre Imarat, Hyderabad, Shri. R. Das,
Director, RF Systems, Dr. V.G. Borkar, Head, Antenna &
Components Group for their consistent support and
encouragement for carrying out the research work.
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[4]
[6]
[7]
[8]
[9]
[10]
. In course of tuning by sliding short, sudden
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Authors
Shri Tapas Kumar Pal, born in October, 1976 at
Kolkata, obtained his M.Sc degree in Physics with
specialization in Microwaves from University of
Calcutta in 1999. He has worked as a Senior
Research Fellow and Senior Scientist at Centre of Advanced
Study in Radiophysics & Electronics, University of Calcutta
for a period of 2.5 years from Feb 2000 onwards. Later he
worked as an Examiner of Patents & Designs at Patent Office,
Kolkata till July 2005. Presently he is working as a Scientist
‘D’ at Research Centre Imarat, Hyderabad. He is currently
engaged in development of MMW Seekers and Sensors. His
special interests are Design and Development of MMW
IMPATT Diodes, Oscillators and Amplifiers and their
computer simulations. He has also been pursuing Ph.D (Tech)
Degree at Institute of Radiophysics & Electronics, University
of Calcutta, on “Millimeter wave IMPATT Diodes and
Oscillators”. He is a Life Member of IETE and Member of
IEEE.
Shri J. V. Prasad obtained his BE in Electronics
and Electrical Engineering from Walchand College
of Engineering Sangli, Maharashtra in 1984 and
ME (Guided missiles) from IAT Pune in 1985. He
is presently working as Sc ‘F’ at RCI and heading the division
Tapas Kumar Pal et. al. / International Journal of Engineering and Technology Vol.2 (5), 2010, 329-335
of Millimeter wave Seekers & Sensors. His areas of interest
are design, development and testing of microwave and
millimeter-wave antennas and their applications in seeker
systems. His special interest is in MMW measurements,
indoor/outdoor system calibration with simulators and
millimeter wave imaging sensors & seekers. He is joint
recipient of 1992 NRDC Independence day award for design
and development of MMW monopulse cassegrain antenna
system. He is a fellow of IETE and a member of Aeronautical
society of India.
Dr J. P. Banerjee obtained B.Sc. (Hons.) and
M.Sc. in Physics and Ph.D. in Radio Physics
and Electronics from the University of Calcutta.
He is presently a full Professor in the Institute
of Radio Physics and Electronics, C.U. He is the recipient of
Indian National Science Academy Award and Griffith
Memorial Prize in Science of the Calcutta University in 1986.
He is the principal co-author of more than 125 research papers
in international journals in the fields of Semiconductor
Science and Technology, Microwave and Millimeter wave
avalanche transit time devices and Avalanche Photo-detectors.
He has authored a text book on solid state electronic devices
published by Vikas Publishing Ltd, New Delhi, India. He is a
fellow of IETE, Senior Member of IEEE, a life member of the
Society of EMI and EMC and Semiconductor Society, India.
He served as a referee for various National and International
technical journals.